I had the test administered by a "certified" assessor, but as the venue was the AYE conference it kind of came free with attendance.

I tested as (surprise, surprise) INTP, although from having had previously used online tests I'd self-identified as INTJ.

really struggle to answer the questions (...) Maybe it is supposed to all come out in the wash.

I'm the same, and yes, it's supposed not to matter. I'd agree with Ben that the test isn't necessarily much more rigorous than a horoscope, but I also agree with Alicorn that the point is more to raise awareness about the existence of more modes of being than the one you're most familiar with. It's a good antidote to generalizing from a sample of size one.